I really think this default should be changed at the qemu level. IIRC qemu
devs were okay with the idea of an adaptive cache default depending on the
underlying storage or image format. qemu/kvm devs are in the most
knowledgeable position to decide these defaults, it shouldn't be pushed up to
libvirt/virt-manager. And if the experts aren't willing to change the default
for block devices, I don't know why virt-manager or libvirt should be expected
to either.
So I would recommend you take your case up with them.

I really don't understand this reasoning. The "experts" are the ones
making the recommendation that libvirt et al are currently ignoring.
Read their website. Caching should be disabled for raw volumes, it's as
simple as that. I'd even hazard to recommend that caching should be
disabled completely unless explicitly selected, but that's only based on
my personal experience thus far. Disabling for raw volumes has been
shown to improve performance significantly in numerous places (I
reference a few in my bug reports). I really don't get what appears to
be a stubborn refusal to make a change that will provide a better
end-user experience.